The preparation of oil-modified phenolic resole varnishes or phenol aldehyde resole varnishes is well known. Typically, a phenol moiety such as phenol, cresylic acid or other similar material and mixtures thereof is mixed with tung or chinawood oil or oiticica oil in the presence of a strong acid such as concentrated sulfuric acid or phosphoric acid, the alkylated oil and then being reacted in a suitable solvent usually alcohol, with an aldehyde moiety in the presence of amine or base material which serves to neutralize the excess strong acid and catalyze the resole formation.
These resole varnishes based on either tung oil or oiticica oil have for many years been used in room temperature punchable paper-based laminates with the NEMA designations XXXPC or FR-2. Illustrative of the state of the art is Claybaker, U.S. Pat. No. 4,043,954, assigned to the assignee of this application, and incorporated herein by reference.
It has been thought that tung oil and oiticia oils are uniquely suitable as modifiers for resoles used as impregnants in room temperature punching laminates with excellent electrical properties, possibly because they possess a high degree of conjugated unsaturation. Recently, however, demand has exceeded supply of these oils and efforts have been made to secure replacements. Surprisingly it has been found that linseed oil can serve to replace all or part of the tung or oiticica oils in such resoles despite the fact that the polyunsaturates in linseed oil are predominately non-conjugated. More specifically, linseed oil is a mixture of triglycerides, isolated by crushing and solvent extraction of flax seed, comprising typically 6% palmitic, 3.5% stearic, 20% oleic, 14.5% linoleic and 56% linolenic glycerides, with a Wijis iodine value of 180-195. The linoleic moiety is the non-conjugated isomer of the conjugated elaeostearic glyceride present in tung oil.
Mention is made of the teachings of Dorland et al, U.S. Pat. No. 2,744,013, in which the suggestion is made that drying oils such as linseed oil, perilla oil, tung oil or mixtures thereof may be used in combination with phenolic resins to make hardboard with enhanced physical strength and moisture resistance. In contrast to the laminating resins with which this invention is concerned, the drying oil component of the Dorland et al system is not used to modify the phenolic resin, but rather is used to impregnate the surface of the hardboard after the resin has been used to coat the wood fibers for consolidation under heat and pressure. Mention is also made of the teachings of Ellis in The Chemistry of Synthetic Resins, Vol. I, Reinhold Publ., 1935, page 402, and particularly footnote 69, which refers to an abstract of German Patent No. 534,784. The abstract describes solid resinous products formed by heat reacting linseed or wood oil with phenols, cresols, etc. and then with formaldehyde in the presence of bases.
In essence, it has been discovered that linseed oil can replace 20-100%, preferably 40-60%, of the tung oil or oiticica oil in resole varnishes now used in the laminating art, with entirely acceptable laminating use properties (stroke cures at 150.degree. C. are in the range of 140.degree.200 seconds). Unexpectedly, paper laminates prepared from the new varnishes have improved properties, especially punchability after thermal aging.